Sealed or Shielded Bearings? How to Tell the Difference

"If there is not a purge path plug, should we assume that it is a sealed bearing?"

Unfortunately, you cannot make any assumptions about bearings being greaseable or sealed by the fittings and pathways alone. A bearing without a purge path does not mean it is sealed. The manufacturer may have designed the bearing to purge through the fill path. For this to be accomplished, you must grease the bearing and then remove the grease fitting to allow excess grease to purge back through the fitting. These bearing operations enable fresh grease to reach the bearing and push out excess grease.

Bearing and motor manufacturers have given various reasons as to why grease fittings are installed on bearings and motors that have sealed bearings, such as: "All motors go down the same assembly line, and it is just easier to put the fittings on every motor that comes down the line." "People would send back motors without grease fittings, as they thought the motor was defective because the fittings were missing." "Their electric motors have no purge port because the design has an internal cavity that will contain all the grease expected to be expelled for the expected motor life."

Generally, it is preferred to see a purge path, as this allows you to verify that the bearing is not overlubricated. It also enables you to purge the old grease when switching lubricant brands or types. Fortunately, most motors and bearings are labeled using either the SKF or Anti-Friction Bearing Manufacturers Association (AFBMA) coding systems. These codes are the best way to determine if a bearing is sealed.

Under the AFBMA system, the code is broken into nine sections: bore diameter, bearing type, duty rating, cage type, shields or seals, snap ring groove, radial clearance, tolerance and lubrication. An AFBMA number might look something like 90BC03JPPN30X.

The SKF system utilizes the same categories but lists them in a different order and identification method: bearing type, duty rating, bore diameter, shields or seals, snap ring groove, cage type, tolerance and radial clearance. An example of an SKF number would be 6318ZZN-C3.

When trying to decide whether a bearing is sealed using the AFBMA or SKF code, note the shields or seals section. If the bearing number uses the AFBMA code, look for "EE" (or "2RSI" for SKF) to determine if the bearing is sealed on both sides. This is the only way to identify whether a bearing is sealed.

Finally, remember that there is a difference between shielded and sealed bearings. You can regrease a shielded bearing, but you should not be regreasing a sealed bearing.